1. Field of the Invention
The present invention relates to a chamber replacing method for use in a multistage amplification laser apparatus having an oscillator and at least one amplifier, chambers of which are common in configuration but have different tolerance limits of deterioration from each other. The present invention particularly relates to a chamber replacing method which is designed such that one of the chambers attached to the oscillator and the one or more amplifiers having a relatively low allowable deterioration limit is detached and reattached to be reused in place of another one of the chambers attached to the oscillator and the one or more amplifiers other than the one having the low allowable deterioration limit.
2. Description of the Related Art
As semiconductor integrated circuits are improved by refining their configuration and increasing the degree of integration, semiconductor exposure devices (hereafter, referred to as “exposure devices”) are required to have improved resolution. For this purpose, studies have been conducted to reduce the wavelength of light emitted by an exposure light source. A gas laser apparatus is used as the exposure light source in place of a conventional mercury lamp. Nowadays, KrF excimer lasers emitting ultraviolet rays having a wavelength of 248 nm and ArF excimer lasers emitting ultraviolet rays having a wavelength of 193 nm are used as exposure gas laser apparatuses.
Studies have been conducted on a next-generation exposure technology referred to as immersion exposure technology in which space between a wafer and an exposure lens of an exposure device has liquid to change the index of refraction, whereby the apparent wavelength of the exposure light source is reduced. When immersion exposure is performed by using an ArF excimer laser as the exposure light source, a wafer is irradiated with ultraviolet light having a wavelength if 134 nm in water. This technology is referred to as ArF immersion exposure technology (or ArF immersion lithography).
One of next-next-generation exposure light sources is an EUV light source. Immersion technology may be performed by using an F2 laser as the exposure light source. In this case, the wafer is irradiated with ultraviolet light having a wavelength of 115 nm.
The lens transmittance is decreased due to increased numerical aperture (NA) according to the immersion exposure technology. Therefore, the output of laser as the light source must be increased in order to achieve fixed exposure. The increase of the laser output is also required to increase the throughput of the exposure device. A double chamber laser apparatus 1 as shown in FIG. 1 is one example of means for obtaining high output with the spectral line width being narrowed. FIG. 1 shows an MOPA laser in which both oscillator and amplifier have a laser resonator.
The double chamber laser apparatus 1 is comprised of an oscillator 10 for outputting narrow-banded laser light, and an amplifier 20 for amplifying the narrow-banded laser beam (referred to as seed light). The double chamber laser apparatus 1 is classified into two types, MOPO type and MOPA type, according to amplification means used in the amplifier 20. The MOPO stands for “master oscillator, power oscillator”, and it is also referred to as an injection lock type laser, in which a resonator is provided before and after an amplifier chamber 21 of the amplifier 20. The MOPA stands for “master oscillator, power amplifier” in which no resonator is provided before or after the amplifier chamber of the amplifier 20.
The following description will be made by way of example of an MOPO laser. The oscillator 10 and the amplifier 20 are each provided with an oscillator chamber 11 and an amplifier chamber 21 for containing laser gas. Windows 11a and 11b are attached to the oscillator chamber 11 to allow free passage of light from inside the chamber to the outside and from outside the chamber to the inside. Windows 21a and 21b are attached to the amplifier chamber 21 to allow free passage of light from inside the chamber to the outside and from outside the chamber to the inside. Further, each of the chambers 11 and 21 is provided therein with a pair of main discharge electrodes, a gas circulation fan, a gas cooler and various other devices. These chambers 11 and 21 may have either same configuration or different configurations.
First, in consideration of functional aspects, it is better that the chambers 11 and 21 be designed independently in optimal manner. The oscillator 10 has functions of generation and band narrowing of laser light, while the amplifier 20 has functions of amplification of laser light output from the oscillator 10. Thus, the oscillator 10 and the amplifier 20 have different functions from each other, and hence it is ideal that the chambers thereof are respectively designed to suit their functions. In this case, the chambers 11 and 21 may possibly not be compatible with each other.
On the other hand, when taking the aspects of cost and management into consideration, it is better that the chambers 11 and 21 have the same configuration. By using common parts for both the oscillator chamber 11 and the amplifier chamber 21, the parts management can be simplified, which makes it easy to reduce the cost for deployment of the parts in servicing centers as well as the production cost.
In some cases, the use of common parts in the oscillator chamber 11 and the amplifier chamber 21 does not incur a significant problem in terms of functions. This is because it is possible to give the oscillator chamber 11 and the amplifier chamber 21 appropriate characteristics for their functions by adjusting parameters such as composition of the gas contained in the oscillator chamber 11 and the amplifier chamber 21, reflectance of a front mirror as a part not belonging to the chambers, and rotation speed of a gas circulation fan. Therefore, it is also possible for laser users to use common parts for both the oscillator chamber 11 and the amplifier chamber 21, placing priority to the advantages in cost and management aspects. The following description is based on the configuration wherein the oscillator chamber 11 and the amplifier chamber 21 have the same configuration.
The oscillator chamber 11 and the amplifier chamber 21 (including their windows and parts inside and outside the chambers) are deteriorated along with the increase of the cumulative operation time of the double chamber laser apparatus 1, or the cumulative number of laser shots. Deterioration of the oscillator chamber 11 and the amplifier chamber 21 induces shortening of the life of the laser. When the degree of deterioration of the chambers is increased, the life of the laser per loading of gas will fail to satisfy the specifications. Therefore, once the degree of deterioration has reached a certain level, the oscillator chamber 11 and the amplifier chamber 21 need be replaced. A tolerance is set for the degree of deterioration of the oscillator chamber 11 and amplifier chamber 21, and it is determined that the chamber has reached the end of its service life once the degree of deterioration reaches the limit, namely the allowable deterioration limit.
FIG. 3 illustrates a concept of the service life of chambers in the oscillator and amplifier. FIG. 3 shows relation between the number of laser shots of the double chamber laser apparatus 1 and the maximum output energy of the oscillator chamber 11 and amplifier chamber 21. The vertical axis represents the chamber output energy, and the horizontal axis represents the cumulative number of laser shots. The maximum output energy can be considered as a capability that the chamber holds at the time. It is known that the capability of the chamber is proportional to the degree of deterioration of the chamber. Accordingly, it can be said that the graph in FIG. 3 also illustrates relation between the number of laser shots and the degree of deterioration of the oscillator chamber 11 and amplifier chamber 21.
As shown in FIG. 3, the maximum output energy of the oscillator chamber 11 and amplifier chamber 21 decreases substantially in proportion to the increase of the number of laser shots. This means that the degree of deterioration of the oscillator chamber 11 and amplifier chamber 21 increases substantially in proportion to the increase of the number of laser shots. It is assumed that the degree of deterioration increases along with the increase of the number of laser shots substantially equally between the oscillator 10 and the amplifier 20. Taking an example of a case in which the service life of the chamber of the oscillator 10 is shorter than that of the chamber of the amplifier 20, when the maximum output energy of a new chamber (the number of laser shots of which is zero) is denoted by A, the degree of deterioration of the oscillator chamber 11 reaches the upper limit of the tolerance, or the allowable deterioration limit and the chamber comes to the end of its service life at the time when the maximum output energy of the oscillator 10 has dropped to B, that is to say, the number of laser shots has reached Lo. As for the amplifier 20, the degree of deterioration of the amplifier chamber 21 reaches the allowable deterioration limit and the chamber comes to the end of its service life at the time when the maximum output energy has dropped to C, that is to say, when the number of laser shots has reached La.
Considering that A in FIG. 3 indicates zero degree of deterioration and taking an example of a case in which the service life of the chamber of the amplifier 20 is shorter than that of the chamber of the oscillator 10, the range of allowable deterioration for the chamber of the oscillator 10 corresponds to the range from A to B, while the range of allowable deterioration for the chamber of the amplifier 20 corresponds to the range from A to C. This means that the allowable deterioration limit of the oscillator 10 is lower than that of the amplifier 20.
In FIG. 3, the relation that 2Lo=La is established. The allowable deterioration limit differs between the oscillator 10 and the amplifier 20 because functions are different between the oscillator 10 and the amplifier 20. The oscillator 10 has functions of generating and narrow-banding laser light, while the amplifier 20 has a function of amplifying the laser light emitted by the oscillator 10. Taking these functions into consideration, it can be seen that the oscillator 10 is required to achieve higher quality of laser oscillation. For this reason, more strict management is required for the oscillator 10 than for the amplifier 20. As a result, the range of allowable deterioration is set smaller, that is the tolerance limit for deterioration is set lower for the oscillator 10. It should be noted, however, that the relation that 2Lo=La is just an example, and this relation is not necessarily established in every case. However, the design can be made such that at least the relation that Lo≦La is established. The setting can be made such that La is an integral multiple of (twice or more) Lo, for example, such that the relation that 2Lo=La or 3Lo=La is established.
Conversely, the allowable deterioration limit is sometimes lower for the chamber of the amplifier 20 than for the chamber of the oscillator 10. When the output energy from the amplifier chamber 21 is increased (when the amplification factor is increased), the amplifier 20 reaches the allowable deterioration limit for the amplifier chamber 21 at the time when the maximum output energy has dropped to B. The oscillator 10 reaches the allowable deterioration limit for the oscillator chamber 11 at the time when the maximum output energy has dropped to C. Since the output energy required for the amplifier chamber 21 is high, the range of allowable deterioration is set small, that is, the allowable deterioration limit is set low. In this case as well, the setting can be made such that Lo is an integral multiple of (twice or more) La, for example, such that the relation that 2La=Lo or 3La=Lo is established.
Description will be made of a conventionally practiced chamber replacing method, taking an example of a case in which the allowable deterioration limit is set lower for the chamber of the oscillator 10 than for the chamber of the amplifier 20.
FIG. 4 illustrates an example of a chamber replacement cycle according to a conventional method (a case in which 2Lo=La). The figures with the symbol “#” in FIG. 4 represent serial numbers of the chambers 11 and 21 used in the oscillator 10 and amplifier 20. When the number of laser shots reaches Lo, the oscillator chamber 11 (#2) of the oscillator 10 is replaced with a new oscillator chamber 11 (#3). When the number of laser shots reaches 2Lo (=La), the amplifier chamber 21 (#1) of the amplifier 20 is replaced with a new amplifier chamber 21 (#4), and the oscillator chamber 11 (#3) of the oscillator 10 is replaced with a new oscillator chamber 11 (#5). Likewise, from then on, every time the number of laser shots is increased by Lo, the oscillator chamber 11 of the oscillator 10 is replaced, and every time the number of laser shots is increased by 2Lo (=La), the amplifier chamber 21 of the amplifier 20 is replaced. The term “new chamber” as used herein means not only a brand-new chamber but also a recycled chamber obtained by overhauling (replacing the parts and cleaning the interior of) a used chamber and having equivalent performance to that of a brand-new one.
When the chambers are replaced according to the replacement cycle shown in FIG. 4, it is determined that the oscillator chamber 11 has reached the allowable deterioration limit for the oscillator 10 every time the number of laser shots is increased by Lo, and the oscillator chamber 11 is overhauled or discarded. However, even if the oscillator chamber 11 has reached the allowable deterioration limit for the oscillator 10, it has not reached the allowable deterioration limit for the amplifier 20. This means that the oscillator chamber 11 is still usable when viewing the whole of the double chamber laser apparatus. If the oscillator chamber 11 is nevertheless overhauled or discarded, it is a waste of labor to overhaul the same and waste of replaced parts.
The present invention has been made in view of such problems, and it is an object of the present invention to enable, in a multistage amplification laser apparatus having an oscillator and at least one amplifier, efficient use of chambers of the oscillator to thereby reduce the labor and parts consumed by replacement of the chambers (to reduce the related cost).